I am making a superhet SSB CW receiver with a crystal filter on the input after the mixer.

Do I have to use IF transformers as the coupling method (1) or can I use capacitor coupling with resistors as the collector load (2) or are RFC better (3)?

Can I use the simple biasing arrangement (B) or the normal bias (A)?

I am aiming for low noise, high sensitivity and ease of construction, this receiver is meant to be reproduced by other people as an relatively easy project.

Is 50 voltage gain per stage good enough? I calculated I need 3 stages for 0.2 µV signal to be translated into a 25 mV signal that will feed the diode mixer detector and AGC detector.

IF is 455kHz. Should I control all the IF stages (AGC to the base) or some different solution?

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    \$\begingroup\$ Show your circuit ideas and please format you question to make it easier to read. \$\endgroup\$
    – Andy aka
    Jan 8, 2020 at 11:09
  • \$\begingroup\$ I can't, the mobile app doesn't take in local photos, says the upload has failed and when giving the photo url it just gives a link. \$\endgroup\$ Jan 8, 2020 at 11:20
  • \$\begingroup\$ I believe this question will be answered best if it is moved to Amateure radio community stack exchange. \$\endgroup\$
    – User
    Jan 8, 2020 at 11:22
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    \$\begingroup\$ gw4sae.wordpress.com/2018/02/26/a-mini-hf-superhet-receiver \$\endgroup\$ Jan 8, 2020 at 11:28
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    \$\begingroup\$ SSB IF amps have a very narrow response and are constructed out of quartz crystals usually where as conventional superhets ones use tuned circuits or a broader response ceramic filters. Also one would have to design separately for different manufacturers crystals as the crystal properties may vary from price to price. One would have to measure or know previously what the series and parallel capacitance values of the crystal. \$\endgroup\$
    – User
    Jan 8, 2020 at 13:24

1 Answer 1


Assumption: the intermediate-frequency crystal filter represents entirely the bandwidth required by the receiver.
You can follow the filter with a wide bandwidth amplifier, with stages coupled by simple coupling capacitors for low-cost reasons. However, you may have to add an extra gain stage. When you use coupling transformers (tuned or untuned), impedance matching allows each stage gain to be higher than capacitor-coupled stage gain. Since transistors, resistors and capacitors are usually cheaper and simpler than transformers, the extra stage is often more attractive.

Identical stages in the I.F. amplifier chain allows automatic gain control to be simultaneously applied to every stage.

I.F. amplifier gain likely represents the bulk of receiver gain. Wideband noise generated within this amplifier may rise above the noise floor at the crystal filter output, especially where you employ very wide-band amplifier stages. In a high-performance receiver, the last I.F. amplifier might include a bandwidth-limiting filter to reduce wideband noise before the detector stage.

A 0.2uV sensitivity requires mentioning the receiver's pass band: a very narrow filter bandwidth makes this an easier task. Noise from the source resistance is involved as well.

  • \$\begingroup\$ I was worried about too much stage gain causing oscillations. I will use an additional audio filter at the output of the detector. Should AGC be applied to the base or the emitter of the transistors? What noise figure of the transistor should I aim for for 0.2uV sensitivity? Is it possible? \$\endgroup\$ Jan 8, 2020 at 16:24
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    \$\begingroup\$ Lots of ways to apply AGC - its your design. And choosing receiver's gain distribution might make I.F. noise figure not relevant...or very relevant. \$\endgroup\$
    – glen_geek
    Jan 8, 2020 at 17:45

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